Exhaust gas treating device

ABSTRACT

A waste gas treatment system has a primary cooling section provided at a stage subsequent to a decomposition treatment section in which a waste gas is subjected to decomposition treatment at high temperature. The primary cooling section has a liquid spray part for spraying a liquid on the treated gas from the decomposition treatment section. A secondary cooling section cools the treated gas sprayed with the liquid in the primary cooling section to reduce the volume of the treated gas. Further, a particle collecting section injects a liquid into the treated gas cooled in the secondary cooling section to collect fine particles contained in the treated gas.

TECHNICAL FIELD

The present invention relates to a waste gas treatment system fortreating a waste gas that is likely to generate dust when it is treatedby combustion. For example, the present invention relates to a waste gastreatment system for treating hazardous and combustible gases containingsilane gas (SiH₄) or a halogen gas (NF₃, CIF₃, SF₆, CHF₃, C₂F₆, CF₄,etc.) discharged from semiconductor manufacturing processes or liquidcrystal panel manufacturing processes, or for treating scarcelydecomposable gases.

BACKGROUND ART

Waste gases that are likely to generate dust include hazardous andcombustible gases, e.g., silane (SiH₄) and disilane (Si₂H₆) dischargedfrom semiconductor manufacturing systems or liquid crystal panelmanufacturing systems, and waste gases containing scarcely decomposable,global warming gases (PFCs). Such waste gases cannot be emitted into theatmosphere as they are. Therefore, the common practice is to introducesuch waste gases into a pretreatment system where flames are formed in afurnace by using an auxiliary burning gas as a combustion gas, such astown (natural) gas or propane gas, and the waste gas is burned in theflames and thus made harmless by oxidation through combustion. Air isusually used as an oxidizing agent for the auxiliary burning gas.

When waste gas is treated at high temperature as stated above, fineparticles (mainly SiO₂) are generated as a by-product. To remove thefine particles, the treated gas is passed through a scrubber (reactiontower) such as that shown in FIG. 14. The scrubber 6 has a cylindricalcasing I in which a pair of packing layers 2 a and 2 b are disposed apredetermined distance apart from each other, and a spray device 5having a spray nozzle 4 at the distal end of a cleaning water piping 3is installed between the packing layers 2 a and 2 b. With the scrubber 6having the described arrangement, treated gas G passing through theinside thereof remains in the space between the packing layers 2 a and 2b, where fine particles are removed with cleaning water sprayed from thespray device 5.

However, fine particles contained in the treated gas G have particlediameters distributed widely over a range of from several nm to severalten μm. Although particles with large diameters can be captured, it hasbeen difficult to remove fine particles not larger than 1 μm.Accordingly, the rate of collection of fine particles has been confinedto about 60 to 70%.

The present invention was made in view of the above-describedcircumstances, and an object of the present invention is to provide awaste gas treatment system designed to be capable of collecting, at ahigh collection rate and with high efficiency, fine particles containedin treated gas resulting from subjecting a waste gas to decompositiontreatment at high temperature.

SUMMARY OF INVENTION

The present invention provides a waste gas treatment system including aprimary cooling section provided at a stage subsequent to adecomposition treatment section in which a waste gas is subjected todecomposition treatment at high temperature, the primary cooling sectionhaving a liquid spray part for spraying a liquid on the treated gas fromthe decomposition treatment section; a secondary cooling section forcooling the treated gas sprayed with the liquid in the primary coolingsection to reduce the volume of the treated gas; and a particlecollecting section for injecting a liquid into the treated gas cooled inthe secondary cooling section to collect fine particles contained in thetreated gas. The particle collecting section uses a fan scrubber havinga rotary impeller. With the waste gas treatment system arranged asstated above, the treated gas is cooled in the secondary cooling sectionto reduce the volume thereof, and fine particles contained in thetreated gas are aggregated to flocs with large particle diameters. Then,a liquid is injected into the treated gas in the particle collectingsection. Therefore, the flocs are captured efficiently. In addition, thevolume of the waste gas to be treated is reduced. Accordingly, it ispossible to increase the rate of removal of particles from the treatedgas and to discharge clean treated gas.

In addition, a U trap is detachably provided in a drain pipe line fordraining water containing foreign matter such as fine particles from theprimary cooling section to collect the foreign matter. Thus, not onlycan the waste gas and waste liquid be separated from each other, butalso blockage of the drain pipe line can be prevented. There istherefore no possibility that waste liquid containing harmful substanceswill overflow. When foreign matter has gathered in the U trap, it isonly necessary to detach the U trap from the drain pipe line and toclean the inside of the U trap. Therefore, maintenance is greatlyfacilitated.

In addition, a sprinkling means for sprinkling water is provided in awaste gas pipe line through which treated gas from the decompositiontreatment section flows. Thus, a flow of water can always be formed onthe inner wall surface of the waste gas pipe line. Therefore, even if atreated gas containing a large amount of dust flows, there is nopossibility of dust adhering to the inner wall surface. Even if thewaste gas G contains a corrosive gas (e.g., HF gas), waste gas pipingwill not be corroded. Further, because water is sprinkled, even if ahigh-temperature treated waste gas flows, the high-temperature treatedwaste gas can be cooled with a minimal amount of water.

In addition, a mist catcher is provided in a waste gas pipe line throughwhich the treated gas from the particle collecting section flows tocatch mist contained in the treated gas. Thus, it is possible to preventmist from flowing out of the system and hence possible to preventcorrosion of piping outside the system and to suppress an increase inpressure loss due to water droplets.

In addition, there is provided a waste gas treatment method wherein awaste gas containing dust and a water-soluble hazardous gas isintroduced into a fan scrubber having a rotary impeller in which thedust and the water-soluble hazardous gas are adsorbed on cleaning waterdroplets to purify the waste gas, wherein the rotary impeller is rotatedat high speed, i.e., not less than 55 m/s in peripheral velocity, andthe amount of cleaning water used is maximized to promote mixing ofwater droplets and the waste gas in the casing of the fan scrubber,whereby the dust and the water-soluble hazardous gas are adsorbed on thecleaning water droplets and thus removed.

In addition, there is provided a waste gas treatment method wherein awaste gas containing hydrophobic dust and a hard-to-dissolve gas isintroduced into a fan scrubber having a rotary impeller in which thehydrophobic dust and the hard-to-dissolve gas are adsorbed on cleaningwater droplets and thus removed to purify the waste gas, wherein therotary impeller is rotated at high speed, i.e., not less than 55 m/s inperipheral velocity, and the amount of cleaning water used is maximizedto reduce the size of water droplets in the casing of the fan scrubberand to increase the density of the water droplets, thereby increasingthe probability that dust particles not larger than 1 μm, in whichrandom motion such as the Brownian motion is dominant, will plungedirectly into cleaning water droplets, and also increasing the area ofcontact between the hard-to-dissolve gas and cleaning water droplets,whereby the hydrophobic dust and the hard-to-dissolve gas are removed topurify the waste gas.

In addition, there is provided a waste gas treatment system wherein awaste gas containing dust and a water-soluble hazardous gas isintroduced into a fan scrubber having a rotary impeller with amultiplicity of blades in which the dust and the water-soluble hazardousgas are adsorbed on cleaning water droplets to purify the waste gas,wherein the rotary impeller has a multiplicity of short blades on anouter peripheral portion thereof. Thus, it is possible to rotate theimpeller at high speed without extra resistance and to increase theamount of cleaning water. Consequently, it becomes possible to reducethe size of cleaning water droplets in the casing and to increase thedensity of the water droplets and hence possible to remove dust andwater-soluble hazardous gas from the waste gas with high efficiency.

In the above-described waste gas treatment system, the rotary impellerof the fan scrubber is provided with a multiplicity of minute holes ineach blade. With this arrangement, it is possible to rotate the impellerat high speed without extra resistance and to increase the amount ofcleaning water. In addition, minute droplets of cleaning water can beemitted through the minute holes of the blades. Consequently, it becomespossible to reduce the size of cleaning water droplets in the casing andto increase the density of the water droplets and hence possible toremove dust and water-soluble hazardous gas from the waste gas with highefficiency.

The rotary impeller may have a multiplicity of short blades on an outerperipheral portion thereof and further have a multiplicity of shortblades and/or a multiplicity of obstacles on an inner peripheral portionthereof. With this arrangement, it is possible to remove dust andwater-soluble hazardous gas from the waste gas with high efficiency asin the case of the above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the whole arrangement of a waste gastreatment system according to an embodiment of the present invention.

FIG. 2 is a vertical sectional view showing a structural example of anevaporator used in the waste gas treatment system shown in FIG. 1.

FIG. 3 is a vertical sectional front view showing a structural exampleof a fan scrubber used in the waste gas treatment system shown in FIG.1.

FIG. 4 is a sectional view taken along the line A—A in FIG. 3.

FIG. 5 is a vertical sectional view showing a structural example of amist catcher used in the waste gas treatment system shown in FIG. 1.

FIG. 6 is a diagram showing the arrangement of a pot-type U trap used inthe waste gas treatment system shown in FIG. 1.

FIG. 7 is a vertical sectional view showing a structural example of apiping spray used in the waste gas treatment system shown in FIG. 1.

FIG. 8 is a diagram showing a structural example of a waste gastreatment system according to the present invention.

FIG. 9 is a vertical sectional view showing a structural example of afan scrubber used in the waste gas treatment system shown in FIG. 8.

FIG. 10 is a diagram showing a structural example of the fan scrubberused in the waste gas treatment system shown in FIG. 8.

FIG. 11 is a vertical sectional view showing a structural example of thefan scrubber used in the waste gas treatment system shown in FIG. 8.

FIG. 12 is a diagram showing a structural example of a waste gascombustor and a liquid spray part used in the waste gas treatment systemshown in FIG. 1.

FIG. 13 is a sectional view as seen in the direction of the arrow I—I inFIG. 12.

FIG. 14 is a sectional view showing a structural example of a scrubber(reaction tower) used in a conventional waste gas treatment system.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below withreference to FIGS. 1 to 7. FIG. 1 shows an example of the wholearrangement of a waste gas treatment system according to an embodimentof the present invention. This treatment system has a waste gascombustor 10 for combustion-treating a waste gas at high temperature asan example of a decomposition treatment section. Herein, the term“decomposition treatment” means that a waste gas is heated to giveenergy thereto so that the waste gas becomes readily decomposable, andair or oxygen is added to the waste gas to oxidize and decompose it.Methods of heating the waste gas include a heater method and acombustion method. With the heater heating method, the waste gas isheated to about 900° C. With the combustion method, the waste gas isburned to heat it to 1300° C. or more. The combustion method imparts alarger amount of heat energy to the waste gas than the heater heatingmethod and is suitable for treating a large amount of waste gas.

The waste gas combustor 10 has a burner part 11 and a combustionreaction part (combustion chamber) 12 for oxidatively decomposing wastegas under heating at a stage subsequent to the burner part 11. Waste gasG is passed through flames formed by the burner part 11, whereby thewaste gas is mixed with the flames and burned. In this embodiment, aburner of the type in which nozzles are opened to supply an auxiliaryburning gas and/or air into a cylindrical space (flame stabilizingportion) as a swirling flow is preferably used as the burner part 11.However, it is also possible to use other appropriate types of burners,as a matter of course. Regarding decomposition treatment, it is alsopossible to use a heater to decompose the waste gas.

The lower end of the waste gas combustor 10 is integrally connected witha liquid spray part 20 for spraying a liquid (mainly water) on thetreated gas, which has been subjected to decomposition treatment in thewaste gas combustor 10. The liquid spray part 20 has a housing 23 havinga nozzle hole 21 in a side thereof. The housing 23 further has a drainoutlet 22 in the bottom thereof. The nozzle hole 21 is connected toliquid supply piping 27 extending from a water-supply pump 25 installedin a circulating water tank 24. The drain outlet 22 is connected to adrain pipe 28.

Waste gas piping 29 is connected to a side of the housing 23 of theliquid spray part 20 to discharge gas from the waste gas combustor 10.The waste gas piping 29 is connected to the upper end of a cooler 40cooled by a refrigerator 30. The cooler 40 has a closed cylindricalcasing 41 in which a central portion in the heightwise direction isformed from a double tube. As shown in FIG. 2, a coiled finned tube 42is disposed in the casing 41. A refrigerant cooled in the refrigerator30 flows through the tube 42. A spray nozzle 45 is provided above thefinned tube 42. The spray nozzle 45 has a nozzle hole 44 opened in thedistal end of cleaning water piping 43. The cooler 40 constitutes anevaporator in which the refrigerant evaporates in the finned tube 42 byabsorbing latent heat from the surrounding gas. In addition, an obstacle46 is provided to allow the waste gas and the finned tube 42 to exchangeheat efficiently. Thus, the efficiency of contact between the waste gasand the finned tube 42 is increased.

The bottom of the casing 41 of the cooler 40 is connected with coolingwater piping 32 for returning cleaning water from the nozzle to thecirculating water tank 24 and gas piping 33 for sending the waste gascooled in the cooler 40 to a fan scrubber 50.

As shown in FIGS. 3 and 4, the fan scrubber 50 has a casing 51 installedin a vertical position. An impeller 52 is disposed in the casing 51 torotate in a vertical plane by the operation of a motor (not shown). Aninlet 53 is opened in the center of one side of the casing 51. The gaspiping 33 is connected to the inlet 53. In addition, water supply piping55 is inserted into the casing 51. The water supply piping 55 has spraynozzle openings 54 for spraying cleaning water radially from the centerof the impeller 52. Meanwhile, the outer peripheral surface of thecasing 51 is provided with a waste gas outlet 56 in the top thereof andalso provided with a drain outlet 57 in the bottom thereof. Further, amist catcher 60 is connected to the top of the casing 51 to catch mistcontained in the waste gas discharged from the fan scrubber 50.

As shown in FIG. 5, the mist catcher 60 has a casing 61 in which baffleplates 62 and 63 are disposed. Each of the baffle plates 62 and 63 isbent at the distal end thereof so as to have an L-shape. The casing 61has a waste gas inlet 64 in the bottom thereof, and a waste gas outlet65 is provided in the top of the casing 61. Further, cleaning waterpiping 67 is disposed above the baffle plate 63. The cleaning waterpiping 67 has a spray nozzle 66 for sprinkling cleaning water. Thecleaning water piping 67 is connected through a valve 35 to piping 34extending from a pump 26. The inlet 64 is connected to the waste gasoutlet 56 of the fan scrubber 50. The waste gas outlet 65 is connectedto an exhaust gas duct (not shown). The piping 34 extending from thepump 26 is connected to the water supply piping 55 of the fan scrubber50. Further, the circulating water tank 24 is supplied with industrialwater from an industrial water source 37. Water is drained from thecirculating water tank 24 to a drain pipe 36 by a blowdown effect.

It should be noted that the system according to this embodiment isprovided with an alkaline liquid tank 70 for supplying an alkalineliquid to the liquid supply piping 27 or 43 according to need. That is,a liquid-supply pump 71 is connected to the alkaline liquid tank 70, andalkaline liquid supply piping 74 and 75 extend from the liquid-supplypump 71 through respective valves 72 and 73. The alkaline liquid supplypiping 74 and 75 join the liquid supply piping 27 or 43. The arrangementis such that by driving the liquid-supply pump 71, the alkaline liquidis added to the liquid sprayed from the nozzle hole 21 of the liquidspray part 20 or from the nozzle hole 44 of the cooler 40.

Next, the treatment of waste gas by the waste gas treatment systemaccording to this embodiment will be described with reference to FIGS.12 and 13. It should be noted that FIG. 12 is a diagram showing astructural example of the waste gas combustor 10 and the liquid spraypart 20 shown in FIG. 1, and FIG. 13 is a sectional view as seen in thedirection of the arrow I—I in FIG. 12. As illustrated in the figures,the side wall of the burner part 11 is provided with an air nozzle 13for injecting air into the burner part 11 to form swirling air flows.The air nozzle 13 has openings provided in the inner peripheral surfacethereof at equal spaces and in a plurality of stages in the verticaldirection. Further, an auxiliary burning gas nozzle 14 is provided belowthe air nozzle 13 to blow off an auxiliary burning gas to form swirlingflows of auxiliary burning gas. The auxiliary burning gas nozzle 14 hasopenings provided in the inner peripheral surface at equal spaces and ina plurality of stages in the vertical direction. Waste gas G to betreated is introduced into the burner part 11 through waste gas inletpipes 15 opened on the lower surface of the top plate of the burner part11. The waste gas G is decomposition-treated at high temperature in thecombustion chamber 12 by swirling flames formed in the burner part 11.Treated gas (combustion gas) G, which has been subjected to thedecomposition treatment, flows into the liquid spray part 20. In theliquid spray part 20, a liquid (mainly water) is sprayed on the treatedgas. Consequently, HCl and HF in the treated gas are absorbed by thesprayed liquid and discharged to the outside through the drain pipe 28.Steam in the treated gas reaches saturation or a state close to it.

The treated gas from which HCl and HF have been removed and in whichsteam has reached saturation or a state close to it flows into thecooler 40. While the treated gas is flowing through the cooler 40,latent heat required for the refrigerant flowing through the finned tube42 to evaporate is taken away from the treated gas. Thus, the treatedgas is cooled rapidly. As the treated gas cools down, the volume of thetreated gas contracts. Consequently, the probability that fine particlesin the treated gas will collide with each other increases. Thus, in thecourse of passing through the cooler 40, fine particles aggregate toflocs with large particle diameters. Treated gas G containing enlargedflocs flows into the fan scrubber 50.

In the fan scrubber 50, while a liquid (mainly water) is being sprayedfrom the spray nozzle openings 54, the impeller 52 is rotated to suckthe treated gas supplied from the inlet 53. The treated gas G is causedto flow around in the casing 51 by the centrifugal action of theimpeller 52 and thus flows into the mist catcher 60 from the waste gasoutlet 56.

In the mechanism for rotating the impeller 52, a motor with an inverteror a DC motor with a rotation control unit is used.

A liquid is sprayed radially from the spray nozzle openings 54 to form acurtain of mist, thereby absorbing enlarged masses of fine particles(flocs) from the treated gas G. The liquid absorbing the flocs collidesagainst the inner peripheral surface of the casing 51. Thereafter, theliquid flows down the inner peripheral surface of the casing 51 andreturns to the circulating water tank 24 from the drain 57 throughpiping 58.

Thus, because fine particles in the treated gas are aggregated to flocswith large particle diameters, it becomes easy to capture them byspraying water or mist. Further, because the volume of the gas to betreated is reduced, fine particles in the treated gas can be collectedefficiently at an increased rate of collection. An experiment carriedout by the inventors of the present application has confirmed that therate of collection of fine particles from the treated gas can beincreased to 90% or more.

The treated gas G flowing into the mist catcher 60 is caused to changeits direction of flow by the baffle plates 62 and 63 as shown by thearrows in FIG. 5. As the direction of flow of the treated gas G ischanged, the mist in the treated gas G cannot effectively change itsflow and collides against the baffle plates 62 and 63. Thus, the mist inthe treated gas G is caught.

By providing the mist catcher 60 downstream the fan scrubber 50 asstated above, the mist can be prevented from flowing out of the system.Thus, it is possible to prevent corrosion of piping outside the systemand to suppress an increase in pressure loss caused by blockage ofpiping with water droplets.

The drain pipe 28 connected to the drain outlet 22 of the housing 23,which is provided at the lower end of the waste gas combustor 10, isprovided with a pot-type U trap 80 for preventing blocking of piping forpassing drain containing foreign matter such as dust. As shown in FIG.6, the pot-type U trap 80 comprises a lid 81 and a pot 82. The lid 81and the pot 82 are connected to each other by a clamp 89. The lid 81 isprovided with a drain inlet 85 and a drain outlet 86. The drain inlet 85is connected with the drain pipe 28 through a ball valve 87. The drainoutlet 86 is connected with the drain pipe 36 through a ball valve 88. Aside of the pot 82 is provided with an observation port 83 for checkingthe amount of foreign matter such as dust collected in the pot 82. It isalso possible to provide a sensor comprising a photoelectric sensor orthe like for detecting that a predetermined amount of foreign matter hasbeen collected, although illustration thereof is omitted.

In the pot-type U trap 80 arranged as stated above, drain containingforeign matter such as dust from the housing 23 flows into the pot 82through the ball valve 87 and the drain inlet 85 and is drained from thepot 82 to the drain pipe 36 through the drain outlet 86 and the ballvalve 88. Although in FIG. 6 the inlet-side drain pipe 28 is higher thanthe outlet-side drain pipe 36, the drain pipe 28 and the drain pipe 36may be at the same height.

Foreign matter M such as dust heavier in specific gravity than waterremains in the pot 82, and only water is drained through the drain pipe36. The foreign matter M such as dust collected in the pot 82 can bechecked as to whether or not a predetermined amount of foreign matterhas been collected from the observation port 83 or by the output of asensor for detecting the foreign matter if it is provided. Therefore,when the amount of foreign matter has reached a predetermined value, theball valves 87 and 88 are closed, and a drain valve 84 is opened todrain water from the pot 82. Thereafter, the clamp 89 is removed, andthe foreign matter is removed from the pot 82. After the inside of thepot 82 has been cleaned, the pot 82 is joined to the lid 81 through theclamp 89. Then, the ball valves 87 and 88 are opened. Thus, drain fromthe housing 23 can be discharged through the pot-type U trap 80 as inthe case of the above. In a case where the above-described sensor isprovided, a control system may be arranged so that the drain valve 84can be opened or closed according to the output of the sensor. That is,the control system may be arranged such that when the sensor detectsthat a predetermined amount of foreign matter has been collected in thepot 82, the drain valve 84 is opened, thereby allowing drainage andcleaning to be carried out automatically.

Thus, provision of the pot-type U trap 80 in the drain pipe 28 allowsforeign matter M such as dust to gather in the pot 82 and makes itpossible to prevent blockage of the drain pipe 36 and the followingpiping with foreign matter. Further, because the pot 82 is joined to thelid 81 simply with the clamp 89, when foreign matter has gathered in thepot 82, the pot 82 can be readily removed from the lid 81 to clean theinside thereof.

Further, because the pot 82 is provided with the observation port 83, itis easy to make a check as to whether or not foreign matter M hasgathered in the pot 82. Further, because the drain valve 84 for drainingwater from the pot 82 is provided, the pot 82 can be removed withoutoverflowing of water containing harmful substances by draining waterbefore the removal of the pot 82.

The drain valve 84 is provided within a range where the observation port83 provided in the side wall of the pot 82 is visible, and immediatelybefore foreign matter M gathers up to the height of the drain valve 84,the pot 82 is removed and cleaned, whereby the pot 82 can be cleanedwithout clogging the drain valve 84.

Further, because the drain pipes 28 and 36 are connected to the draininlet 85 and the drain outlet 86 of the pot-type U trap 80 through therespective ball valves 87 and 88, the pot 82 can be cleaned withoutletting the upstream- and downstream-side atmospheres be released to theatmospheric air when the pot 82 is removed.

A piping spray 90 is provided in a vertical piping portion of each ofthe waste gas piping 29 and 33. As shown in FIG. 7, the piping spray 90comprises a spray nozzle 93 provided in a vertical piping portion ofeach of the waste gas piping 29 and 33. The spray nozzle 93 has a nozzlehole 92 opened at the distal end of cleaning water piping 91. Thecleaning water piping 91 is connected to the liquid supply piping 27 andthe piping 34 and supplied with water.

In the piping spray 90 arranged as stated above, water is alwayssprinkled from the spray nozzle 93 at a flow rate, for example, of 1l/min. Consequently, water W flows down the inner wall surfaces of thewaste gas piping 29 and 33, i.e., a water stream layer is formed on theinner wall surfaces. Therefore, even if treated waste gas G containing alarge amount of dust flows, there is no possibility of dust adhering tothe inner wall surfaces. Further, because water flows down the innerwall surfaces of the waste gas piping 29 and 33 by being sprinkled atall times, the waste gas piping is not corroded even if the treatedwaste gas G contains a corrosive gas (e.g., HF gas). Further, becausewater is sprinkled therein, the waste gas piping 29 and 33 are filledwith water droplets. Accordingly, even if high-temperature treated wastegas G flows in, because the treated waste gas G contacts the waterdroplets for a long period of time, the high-temperature treated wastegas G can be cooled with a minimal amount of water. In addition, thecooling efficiency in the cooler 40 can also be increased. Even iftreated gas G is arranged to flow opposite to the flow of water sprayedfrom the nozzle hole 92 of the spray nozzle 93 (i.e., upward from thebottom as viewed in the figure), the same effect can be obtained, as amatter of course.

In a case where the piping spray 90 is not provided in the verticalportion of each of the waste gas piping 29 and 33 as stated above, it isnecessary to perform maintenance on the waste gas piping 29 and 33 oncea month. However, by installing the piping spray 90 having theabove-described arrangement, it becomes necessary to perform maintenanceonly once every 6 months. It should be noted that the piping spray is,basically, used in a vertical portion of piping, that is, in verticalpiping. However, where the piping spray is installed is not limitedthereto; it is also usable in obliquely inclined piping or horizontallyinstalled piping. In such a case, the same effect can also be obtained.

In the waste gas treatment system arranged as stated above, the flowvelocity at which waste gas G1 is introduced into the burner part 11 ofthe waste gas combustor 10 is made higher than the hydrogen flamepropagation velocity (2.5 to 2.8 n/s) in the atmospheric air by reducingthe bore diameter of the waste gas inlet pipe, thereby preventing theoccurrence of backfire into the waste gas inlet piping. A pressure lossdue to the flow velocity control effected by reducing the bore diameterof the waste gas inlet pipe is canceled by keeping the inside of thesystem at a negative pressure at all times through the fan scrubber 50.

Further, keeping the inside of the system at a negative pressure at alltimes through the fan scrubber 50 makes it possible to effectivelyreduce the load on a vacuum pump at the stage preceding the waste gastreatment system and also makes it possible to prevent waste gas fromleaking to the outside if there is a pinhole or the like in pipingextending between the vacuum pump and the waste gas treatment system orin a gas contact portion in the waste gas treatment system.

Further, by-product collecting performance at a primary cooling sectioncomprising the liquid spray part 20, which is provided at the stagesubsequent to the waste gas combustor 10, is confined to roughcollection of high-concentration acid gases and fine particles. Theliquid (mainly water) used in the liquid spray part 20 is not returnedto the circulating path in the system but drained to the outside of thesystem as primary cooler drain, thereby reducing the amount of waterdrained from the circulating water tank 24 by a blowdown effect.

Further, the cooler 40, which is temperature-controlled by therefrigerator 30, is provided as a secondary cooling section at a stagesubsequent to the primary cooling section. Thus, when treated waste gasG is passed through the cooler 40, heat exchange with the refrigerant isperformed, whereby the particle diameter of fine particles can beincreased by the condensation of gas, and the treated waste gas G can becooled. Furthermore, water is sprayed from the spray nozzle 45 in thecooler 40, thereby collecting and removing acid gases remaining in traceamounts and reducing the temperature of sprayed water at the same time.Thus, it is possible to solve the problem of a rise in temperature ofcirculating water in the waste gas treatment system and hence possibleto reduce the amount of blowdown water from the circulating water tank24. As a result, the amount of cooling and cleaning water used in thesystem can be reduced by a considerable extent.

Further, the waste gas treatment system employs the fan scrubber 50having the impeller 52 therein to perform contact mixing of treatedwaste gas and cleaning water, thereby increasing the gas-liquid contactefficiency and achieving a reduction in size of the system as well asreducing the amount of water used. Further, because the particlediameter of fine SiO₂ particles is increased in the secondary coolingsection, particle removal by dust collection is further facilitated.

FIG. 8 is a diagram showing a structural example of a waste gastreatment system using a fan scrubber according to the presentinvention. In this waste gas treatment system, as illustrated in thefigure, waste gas from the liquid spray part 20 provided at the lowerend of the waste gas combustor 10 is introduced directly to a fanscrubber 100. The fan scrubber 100 has an impeller 102 rotated by amotor 101 for high-speed rotation. Water supply piping 104 is disposedin the fan scrubber 100. The water supply piping 104 has a spray nozzle103 for sprinkling cleaning water radially from the center of theimpeller 102. A mist catcher 105 is installed in the upper part of thefan scrubber 100. Drain piping 106 is disposed at the lower end of thefan scrubber 100.

In the waste gas treatment system arranged as stated above, waste gasdischarged from the liquid spray part 20 at the lower end of the wastegas combustor 10 contains dust, e.g., SiO₂, and water-soluble hazardousgases. The impeller 102 is rotated at high speed, and the amount ofcleaning water blown off from the spray nozzle 103 is increased, therebypromoting mixing of cleaning water droplets and waste gas in the casing107 of the fan scrubber 100. Consequently, dust and water-solublehazardous gases in the waste gas are effectively adsorbed on thecleaning water droplets. Thus, the dust and hazardous gases are removedfrom the waste gas with high efficiency. In a waste gas containing finehydrophobic dust particles with a particle diameter not larger than 1 μmand somewhat hard-to-dissolve gases, in particular, the motion of dustparticles is such that random motion such as the Brownian motion is moredominant than unitary motion of dust as particles of a gaseous body.Therefore, by utilizing this fact, the impeller 102 is rotated at highspeed, and the amount of cleaning water is increased, whereby waterdroplets filling the casing 107 are reduced in size and increased indensity, thereby increasing the area of contact between the cleaningwater and the waste gas. Thus, fine dust particles performing randommotion and somewhat hard-to-dissolve gasses are readily adsorbed oncleaning water droplets.

In a case where the number of revolutions of an impeller with a diameterof 220 mm was set at 4800 rpm or more (peripheral velocity: 55 m/s ormore) and the amount of cleaning water was increased such that the powerof the high-speed rotation motor 101 (rating: 0.7 kw) was 90 to 100% (asthe amount of cleaning water is increased, the load on the high-speedrotation motor 101 also increases owing to an increase in resistance),it was confirmed that it was possible to remove dust and somewhathard-to-dissolve gases with high efficiency, i.e., 90% for dustparticles not larger than 1 μm, and 99% for such gases. Thus, dust andhazardous gases in waste gas can be removed with high efficiency(inferior in efficiency to a filter), without a likelihood of cloggingas occurs in a filter, by rotating the rotary impeller of the fanscrubber 100 at high speed and increasing the amount of cleaning water.

As the impeller 102, an impeller having a structure as shown in FIG. 9is used. That is, a multiplicity of blades 108 having a short length Lare provided on the outer peripheral portion where the flow velocity ishigh and the impact effect is large. Thus, air resistance and so forthare reduced, and extra resistance disappears. Therefore, the amount ofcleaning water blown off from the spray nozzle 103 can be increasedcorrespondingly, and the impeller 102 can be rotated at correspondinglyhigh speed. As a result, it becomes possible to reduce the size ofcleaning water droplets in the casing 107 and to increase the density ofcleaning water droplets and hence possible to remove dust andwater-soluble hazardous gases from waste gas with high efficiency.

Further, as the impeller 102, an impeller having a multiplicity ofminute holes 108 a in each blade 108, as shown in FIG. 10, may be used.By providing a multiplicity of minute holes 108 a in each blade 108, airresistance and so forth are reduced, and minute droplets of cleaningwater can be emitted from the reverse side of each blade 108 through theminute holes 108 a. As a result, it becomes possible to rotate theimpeller 102 at high speed, reduce the size of cleaning water dropletsin the casing 107 and increase the density of cleaning water dropletsand hence possible to remove dust and water-soluble hazardous gases fromwaste gas with high efficiency.

Further, as the impeller 102, an impeller arranged as shown in FIG. 11may be used. That is, a multiplicity of blades 108 are installed on theouter peripheral portion, and a multiplicity of short blades 109 androd-shaped obstacles 110 are discontinuously disposed on the innerperipheral side of the blades 108. By providing the blades 109 and theobstacles 110 on the inner peripheral side of the blades 108 arrayed onthe outer peripheral portion as stated above, the flow of waste gaspassing through the impeller 102 is made turbulent to promote mixing ofthe waste gas with cleaning water droplets. Further, minute waterdroplets are produced when cleaning water blown off from the spraynozzle 103 collides with the blades 108 and 109 and the obstacles 110.Thus, it becomes possible to reduce the size of cleaning water dropletsin the casing 107 and to increase the density of cleaning water dropletsand hence possible to remove dust and water-soluble hazardous gases fromwaste gas with high efficiency.

It should be noted that the impeller 102 may be arranged such that amultiplicity of blades 108 are installed on the outer peripheral portionand only a multiplicity of short blades 109 are discontinuously disposedon the inner peripheral side of the blades 108, although illustration ofthis arrangement is omitted.

Further, the above-described fan scrubber operating method and theimpeller structures shown in FIGS. 9 to 11 may be used for the fanscrubber 50 of the waste gas treatment system shown in FIG. 1 and forthe impeller of the fan scrubber 50. By doing so, dust and hazardousgases in waste gas can be removed with higher efficiency.

INDUSTRIAL APPLICABILITY

As has been stated above, according to the present invention, a wastegas is subjected to decomposition treatment at high temperature andsprayed with a liquid in a primary cooling section so as to be cooled.The treated waste gas is cooled in a secondary cooling section to reducethe volume of the treated gas, and fine particles contained in thetreated gas are aggregated to flocs with large particle diameters. Then,a liquid is injected into the treated gas in a particle collectingsection. Therefore, the flocs are captured efficiently. In addition, thevolume of the waste gas to be treated is reduced. Accordingly, it ispossible to increase the rate of removal of particles from the treatedgas and to discharge clean treated gas.

In addition, according to the invention, a U trap is detachably providedin a drain pipe line. Therefore, blockage of the drain pipe line can beprevented. When foreign matter has gathered in the U trap, it is onlynecessary to detach the U trap from the drain pipe line and to clean theinside of the U trap. Therefore, maintenance is extremely facilitated.

In addition, according to the invention, a water sprinkling means isprovided in a waste gas pipe line. Therefore, a flow of water can alwaysbe formed on the inner wall surface. Even if a treated gas containing alarge amount of dust flows, there is no possibility of dust adhering tothe inner wall surface. Even if the waste gas G contains a corrosive gas(e.g., HF gas), the waste gas piping will not be corroded. Further,because water is sprinkled, even if a high-temperature treated waste gasflows in, the high-temperature treated waste gas can be cooled with aminimal amount of water.

In addition, according to the invention, a sprinkling means forintermittently or continuously sprinkling water is provided in the wastegas piping of the secondary cooling section. Therefore, even if thewaste gas G contains a corrosive gas, the waste gas piping will not becorroded. Further, because water is sprinkled, even if ahigh-temperature treated waste gas flows in, the high-temperaturetreated waste gas can be cooled with a minimal amount of water.

In addition, according to the invention, a mist catcher is provided atthe downstream side of the particle collecting section. Therefore, it ispossible to prevent mist from flowing out of the system and hencepossible to prevent corrosion of piping outside the system and tosuppress an increase in pressure loss due to water droplets.

In addition, according to the invention, a liquid used in the waste gastreatment system, exclusive of the primary cooling section, is reused.Therefore, it is possible to reduce the amount of liquid used in thewaste gas treatment system.

In addition, according to the invention, mixing of the waste gas andcleaning water droplets in the fan scrubber casing is promoted, so thatdust and water-soluble hazardous gases in the waste gas can be removedwith high efficiency.

In addition, according to the invention, the waste gas and waterdroplets in the fan scrubber casing are reduced in size and the densityof the water droplets is increased. Accordingly, hydrophobic dustparticles in the waste gas that have a particle diameter not larger than1 μm and hard-to-dissolve gases can be removed with high efficiency.

In addition, according to the invention, the rotary impeller has amultiplicity of short blades on an outer peripheral portion thereof.Therefore, it is possible to rotate the impeller at high speed withoutextra resistance and to increase the amount of cleaning water.Consequently, it becomes possible to reduce the size of cleaning waterdroplets in the casing and to increase the density of the water dropletsand hence possible to remove dust and water-soluble hazardous gases fromthe waste gas with high efficiency.

In addition, according to the invention, the rotary impeller has amultiplicity of minute holes in each blade. Therefore, it is possible torotate the impeller at high speed without extra resistance and toincrease the amount of cleaning water. In addition, minute droplets ofcleaning water can be emitted through the minute holes of the blades.Consequently, it becomes possible to reduce the size of cleaning waterdroplets in the casing and to increase the density of the water dropletsand hence possible to remove dust and water-soluble hazardous gases fromthe waste gas with high efficiency.

The rotary impeller can also have a multiplicity of short blades on anouter peripheral portion thereof and further has a multiplicity of shortblades and/or a multiplicity of obstacles on an inner peripheral portionthereof. Therefore, the flow of waste gas passing through the impelleris made turbulent to promote mixing of the waste gas with cleaningwater. Further, it is possible to remove dust and water-solublehazardous gases from the waste gas with high efficiency by minute waterdroplets produced when cleaning water collides with the multiplicity ofshort blades and/or the multiplicity of obstacles.

What is claimed is:
 1. A waste gas treatment method comprising: after awaste gas has been subjected to decomposition treatment at hightemperature in a decomposition treatment section, spraying the treatedgas subjected to the decomposition treatment with a liquid in a primarycooling section to cool the treated gas; cooling the treated gas in asecondary cooling section with a cooler cooled by a refrigerator toreduce a volume of the treated gas; and collecting fine particlescontained in the treated gas in a particle collecting section.
 2. Atreatment system comprising: a primary cooling section provided at astage subsequent to a decomposition treatment section in which a wastegas is subjected to decomposition treatment at high temperature, saidprimary cooling section having a liquid spray part operable to spray aliquid on the treated gas from the decomposition treatment section; asecondary cooling section having a cooler cooled by a refrigerator, saidsecondary cooling section being operable to cool the treated gas sprayedwith the liquid in said primary cooling section to reduce a volume ofthe waste gas; and a particle collecting section operable to inject aliquid into the treated gas cooled in said secondary cooling section tocollect fine particles contained in the treated gas.
 3. A treatmentsystem according to claim 2, wherein said particle collecting section isa fan scrubber having a rotary impeller.
 4. A waste gas treatment methodaccording to claim 1, wherein the particle collecting section comprisesa fan scrubber having a casing and a rotary impeller, and said waste gastreatment method further comprises introducing the treated gascontaining dust and a water-soluble hazardous gas into the fan scrubberwhere the dust and water-soluble hazardous gas are adsorbed on cleaningwater droplets to purify the waste gas, wherein the rotary impeller isrotated at a high speed and an amount of cleaning water used ismaximized to promote mixing of the cleaning water droplets and thetreated gas in the casing of fan scrubber, whereby the dust andwater-soluble hazardous gas are adsorbed on the cleaning water dropletsand thus removed.
 5. A waste gas treatment method according to claim 1,wherein the particle collecting section comprises a fan scrubber havinga casing and a rotary impeller, and said waste gas treatment methodfurther comprises introducing the treated gas containing hydrophobicdust and a hard-to-dissolve gas into the fan scrubber where thehydrophobic dust and hard-to-dissolve gas are adsorbed on cleaning waterdroplets and thus removed to purify the waste gas, wherein the rotaryimpeller is rotated at a high speed and an amount of cleaning water usedis maximized to reduce a size of the cleaning water droplets in thecasing of the fan scrubber and to increase a density of the cleaningwater droplets, thereby increasing a probability that particles of thehydrophobic dust not larger than 1 mm, in which random motion such asBrownian motion is dominant, will plunge directly into the cleaningwater droplets, and also increasing an area of contact between thehard-to-dissolve gas and the cleaning water droplets, whereby thehydrophobic dust and hard-to-dissolve gas are removed to purify thewaste gas.
 6. A waste gas treatment system according to claim 2, whereinthe waste gas contains dust and a water-soluble hazardous gas and saidparticle collecting section comprises a fan scrubber having a rotaryimpeller with a plurality of short blades located at an outer peripheryof said rotary impeller, said fan scrubber being operable to cause thedust and water-soluble hazardous gas to be adsorbed on cleaning waterdroplets to purify the waste gas.
 7. A waste gas treatment systemaccording to claim 2, wherein the waste gas contains dust and awater-soluble hazardous gas and said particle collecting sectioncomprises a fan scrubber having a rotary impeller with a plurality ofblades with a plurality of minute holes located therein, said fanscrubber being operable to cause the dust and water-soluble hazardousgas to be adsorbed on cleaning water droplets to purify the waste gas.8. A waste gas treatment system according to claim 2, wherein thetreated gas contains dust and a water-soluble hazardous gas and saidparticle collecting section comprises a fan scrubber having a rotaryimpeller with a plurality of short blades located at an outer peripheryof said rotary impeller and at least one of an additional plurality ofshort blades and a plurality of obstacles located at an inner portion ofsaid rotary impeller, said fan scrubber being operable to cause the dustand water-soluble hazardous gas to be adsorbed on cleaning waterdroplets to purify the waste gas.
 9. A waste gas treatment systemcomprising a decomposition treatment section operable to subject a wastegas to decomposition treatment at high temperature, and a treatmentsystem, said treatment system comprising: a primary cooling sectionprovided at a stage subsequent to said decomposition treatment section,said primary cooling section having a liquid spray part operable tospray a liquid on the treated gas from said decomposition treatmentsection; a secondary cooling section having a cooler cooled by arefrigerator, said secondary cooling section being operable to cool thetreated gas sprayed with the liquid in said primary cooling section toreduce a volume of the treated gas; a particle collecting sectionoperable to inject a liquid into the treated gas cooled in saidsecondary cooling section to collect fine particles contained in thetreated gas.
 10. A waste gas treatment system according to claim 9,wherein said particle collecting section is a fan scrubber having arotary impeller.
 11. A waste gas treatment system according to claim 9,further comprising a U trap detachably provided in a drain pipe line fordraining water containing foreign matter from said primary coolingsection, said U trap being operable to collect the foreign matter.
 12. Awaste gas treatment system according to claim 9, further comprisingsprinkling means for sprinkling water provided in a waste gas pipe linethrough which the treated gas from said decomposition treatment sectionflows.
 13. A waste gas treatment system according to claim 11, furthercomprising sprinkling means for sprinkling water provided in a waste gaspipe line through which the treated gas from said decompositiontreatment section flows.
 14. A waste gas treatment system according toclaim 9, further comprising sprinkling means for intermittently orcontinuously sprinkling water provided in waste gas piping of saidsecondary cooling section.
 15. A waste gas treatment system according toclaim 11, further comprising sprinkling means for intermittently orcontinuously sprinkling water provided in waste gas piping of saidsecondary cooling section.
 16. A waste gas treatment system according toclaim 12, further comprising sprinkling means for intermittently orcontinuously sprinkling water provided in waste gas piping of saidsecondary cooling section.
 17. A waste gas treatment system according toclaim 9, further comprising a mist catcher provided in a waste gas pipeline through which the treated gas from said particle collecting sectionflows, said mist catcher being operable to catch mist contained in thetreated gas.
 18. A waste gas treatment system according to claim 11,further comprising a mist catcher provided in a waste gas pipe linethrough which the treated gas from said particle collecting sectionflows, said mist catcher being operable to catch mist contained in thetreated gas.
 19. A waste gas treatment system according to claim 12,further comprising a mist catcher provided in a waste gas pipe linethrough which the treated gas from said particle collecting sectionflows, said mist catcher being operable to catch mist contained in thetreated gas.
 20. A waste gas treatment system according to claim 14,further comprising a mist catcher provided in a waste gas pipe linethrough which the treated gas from said particle collecting sectionflows, said mist catcher being operable to catch mist contained in thetreated gas.
 21. A waste gas treatment system according to claim 9,wherein the liquid used in said waste gas treatment system, exclusive ofsaid primary cooling section, is reused.
 22. A waste gas treatmentsystem according to claim 11, wherein the liquid used in said waste gastreatment system, exclusive of said primary cooling section, is reused.23. A waste gas treatment system according to claim 12, wherein theliquid used in said waste gas treatment system, exclusive of saidprimary cooling section, is reused.
 24. A waste gas treatment systemaccording to claim 14, wherein the liquid used in said waste gastreatment system, exclusive of said primary cooling section, is reused.25. A waste gas treatment system according to claim 17, wherein theliquid used in said waste gas treatment system, exclusive of saidprimary cooling section, is reused.
 26. A waste gas treatment methodaccording to claim 4, wherein the rotary impeller is rotated at not lessthan 55 m/s in peripheral velocity.
 27. A waste gas treatment methodaccording to claim 5, wherein the rotary impeller is rotated at not lessthan 55 m/s in peripheral velocity.
 28. A waste gas treatment systemaccording to claim 11, wherein the foreign matter is fine particles andsaid U trap is operable to collect the fine particles.